1. Field of the Invention
This invention relates to a method of forming a metal plate pattern, such as a lead frame or a metal mask mesh or a wiring pattern on a printed circuit board, or, in particular, to a method of forming a high aspect ratio fine metal plate pattern, such a lead frame or a metal mask mesh, from a metal plate using semiadditive process pattern forming technique or a method of forming a fine wiring pattern on an insulating substrate to fabricate a printed circuit board.
2. Description of the Related Art
The subtractive process is an inexpensive, simple method of fabricating a printed circuit board and has been most widely used. on the other hand, the recent trend toward a higher density and a smaller size of semiconductor devices and various electronic devices, the subtractive method is disadvantageous in some points when producing fine conductor patterns on a circuit board.
A method of forming a metal pattern has been proposed in which the etching process is provisionally suspended after being etched to a predetermined depth along the thickness of an etched layer and the side etched portion generated by the first etching session is covered by an anti-etching layer, after which the etching process is resumed.
A metal is etched in a plurality of stages to secure a high aspect ratio in any of the conventional techniques described below.
(1) The layer to be etched is coated with a dry film resist (DFR) as a mask, which is patterned by exposure and development, after which the layer to be etched is etched (by “half etching”). The term of “half etching” or “selective etching” in this specification means an etching by which the layer to be etched is not fully etched through the thickness thereof, but etched until a predetermined thickness thereof. The resulting side etched portion is protected by an anti-etching layer, and the etching process executed again thereby to produce a high-density pattern (Japanese Unexamined Patent Publication Nos. 1-188700 and 1-290289, for example). In this case, the use of a positive photosensitive resist (Japanese Unexamined Patent Publication No. 10-229153, for example) or an electrodeposit resist (Japanese Unexamined Patent Publication No. 2004-204251, for example) has been proposed as an anti-etching protective layer for the side etched portion.
These conventional techniques pose a problem, however, in that the light masking property of the DFR forming the anti-etching protective layer of the side etched portion is insufficient and therefore the positive resist under the DFR is melted and generates a gap with the DFR, resulting in the loss of the function as an anti-etching protective layer. Another problem is that the DFR is melted (expanded) and separated by the developer or the adhesion of the DFR is reduced and the DFR is separated by deformation under the liquid pressure of the developer during the development of the positive resist, resulting in the loss of the mask function.
(2) As described above, a method has been proposed in which the DFR is patterned by lamination as a mask on the etched layer, after which the light masking property of the DFR is improved by forming a toner layer as a masking layer on the DFR and the etching process is conducted in a plurality of stages (Japanese Unexamined Patent Publication No. 2005-026646, for example). Also in this case, the DFR is melted (expanded) and separated by the developer during the development of the positive resist or the deformation of the DFR under the liquid pressure of the developer reduces the adhesion of the DFR, which is thus separated.
(3) As in (2) above, a method has been proposed (in Japanese Unexamined Patent Publication No. 2005-026645, for example) in which the etching process is executed in a plurality of stages after forming a thin metal (silver) layer between the etched layer and the DFR to improve the light masking property of the DFR. This method also poses a problem that the DFR is melted (expanded) and separated by the developer during the development of the positive resist, and the liquid pressure of the developer deforms and damages the thin metal layer resulting in a loss of the mask function.